Removal of hydrogen fluoride from gases



2,813,000 REMOVAL OF HYDROGEN FLUORIDE FROM GASES Richard C. Quittenton,Arvida, Quebec, Canada, assignor to Aluminium Laboratories Limited,Montreal, Quebec, Canada, a corporation of Canada Application March 31,1954, Serial No. 420,167

8 Claims. (Cl. 23-2) This invention relates to the recovery of fluorinevalues from'gases and more particularly to the removal of hydrogenfluoride from waste gases or the like in which it is present, usually inminor proportion. A particular aim of the invention is to provide aprocedure 'that'willbe highly economical and yet will permit convenientrecovery of fluoride compounds for use.

' -In industrial processes where discharged gases contain appreciablequantities of hydrogen fluoride, it is usually desirable to treat thegas for removal or reduction of the hydrogen fluoride content, bothbecause the release of such substance at rates above a selected minimummay sometimes be objectionable as detrimental to animal or other life,and because valuable fluorides can be recovered by such treatment. Forexample, scrubbing procedures have commonly been used for reducing thehydrogen fluoride content of the gases withdrawn from or over aluminumreduction cells, i. e. in plants where metallic aluminum is produced byelectrolysis of a fused bath containing alumina.

Among various known methods for such treatment of gases, scrubbing withlarge quantities of plain water can be fairly elfective in reducing thehydrogen fluoride content. Since in such case the dissolved fluoride canonly be made to reach a relatively low concentration, its actualrecovery from the scrubbing water is usually too costly for practicalpurposes. Another method has involved the use of solid basic substancessuch as lime packed in towers, but operations of this type have notappeared efficient for treatment of aluminum reduction cell gas(so-called potroom gases), a particular difliculty being of a physicalnature in handling the gas, i. e. because of the back pressure involvedin trying to drive it through the packed towers.

A third practice, perhaps the most common, has been to scrub or reactthe gas with an alkaline solution, such as an alkaline solution ofsodium carbonate or other base or basic salt. from the waste gases isobtained, and the process permits ultimate recovery of fluoride as thesodium or other saltv corresponding to the alkaline compound employed.One.

mode of operation, for instance, is to permit the fluorine salts tobuild up to a relatively high value in solution in the alkalinescrubbing liquor. Then after filtering to remove carbon andmiscellaneous other solids which were carried by the gases, the clearsolution is treated in socalled gassing towers, where carbon dioxide isapplied. This treatment, which acidifies the solution, causesprecipitation of the fluoride salts, e. g. as sodium fluoride, or ifsodium aluminate has been added, as cryolite.

While these alkaline scrubbing processes are effective, they have beenrather expensive when used in certain industries such as aluminumreduction plants. The potroom gases of the latter not only containhydrogen fluoride, other gases such as carbon monoxide and dioxide, somesolids or dust, e. g. alumina and carbon, and perhaps other fluorides ingaseous or solid form, but also very commonly include sulfur dioxide inamount com- Good removal of hydrogen fluoride parable to the fluorine(or HF) content, indeed often up to two or more times the quantity ofthe latter. The sulfur dioxide is taken up very thoroughly by thescrubbing solution, utilizing large amounts of soda or other alkali, upto several times the amount required by the HF, in the formation ofsulfite or sulfate salts. This heavy consumption of soda is a seriouswaste, since the resulting sulfites or sulfates are of insignificantvalue for recovery, as compared with the fluorides. Furthermore, thelarge concentration of sulfates or the like in the scrubbing liquorcorrespondingly complicates the ultimate fluoride separation, e. g. inbeing one of the reasons for requiring the expensive CO2 gassing step.

The present invention is therefore designed to provide a more economicaland eflicient method of fluorine removal and recovery, especially byreducing the excess consumption or loss of alkaline reagent in thehandling of gases containing significant amounts of sulfur dioxide.

For the attainment of improvement in the foregoing and other importantrespects, the new process of the invention involves, as an essentialstep, the scrubbing or other treatment of the fluoride-containing gaswith an aqueous solution that is maintained in an acid condition, e. g.at

no higher than a readily determinable amount for which the HFequilibrium concentration in the scrubbed gas is at or below the reducedHF value sought; and (2) that this condition can be achieved by takingup the dissolved HF with the fluoride-reactive substance (i. e. the

neutralizing ions) in the solution, while restricting theconcentrationof such substance to keep the pH at or within a correspondingly selectedvalue or range, prefer ably below 5.5.

It has been found that under these circumstances, i. e. by utilizing anacid (rather than an alkaline) scrubbing liquor While neutralizing theabsorbed hydrogen fluoride continuously to keep its free concentrationbelow a selected value, the simultaneous absorption of sulfur dioxide isreduced to a relatively low, indeed very minor extent. Thus in effecthydrogen fluoride is separated from the gas selectively relative tosulfur dioxide, with consequent saving in use of soda or other reagent;in many cases, the reagent requirement is only a minor fraction of whatis needed for conventional alkaline scrubbing. At the same time, highefliciency of fluoride removal is easily achieved, and recovery offluoride from the solution is greatly facilitated, being in many casesessentially automatic, with no need for the gassing towers heretoforeemployed.

In carrying out the process, the waste gases, for example after removalof solids, are passed through a Patented Nov. 12, 1957 srubber wherethey are brought into intimate and extended contact with the liquid, e.g. in sprays of conventional character. From the beginning, the liquoris made to contain a certain proportion of the neutralizing orfluoride-reacting material, such as sodium carbonate,

preferably in amount insuflicient to provide a strongly alkaline pH evenbefore any HP is absorbed. As the hydrogen fluoride dissolves insuccessive quantities of the liquid, the concentration of soda ispromptly reduced by reaction, as by formingsodium fluoride, anddissolved hydrogen fluoride tends to build up, depressing the pH. Theprocess is advantageously continuous with the liquor recirculated andsoda added continuously or from time to I time as may be necessary tokeep the pH in a desired range, usually about 3.5 to 4.0.

.Under the acid conditions of the liquor, the sodium fluoride has only arelatively low solubility so that it soon commences to precipitate out.Thus in normal operation with soda as the reagent, the discharge fromthescrubber will consist of a slurry, the solids being chiefly sodiumfluoride. The recirculating system may include a settling tank, wherethese solids are removed, while the clear liquor returns to re enter thescrubber as spray. Although a certain concentration of dissolved sulfurdioxideis builtup and maintained in the .liquor, it is essentially asaturation value and relatively little sulfur saturation of dissolvedS02 has been reached. As the process continues, there can beconveniently continuous remfovaljof :sodiurn fluoride or other desiredfluoride. Soda solution and make-up water are added as necessary,thesoda addition being preferably limited or controlled by .a. singlefactor, namely to keep the soda content as highas possible whilemaintaining an acid pH. Preferredcontrol is to maintain or approximate aselected pH, chosen in the range from above 1 or 1.5 up to 5.5. As willbe. further explained, other fluoride compounds canbe prepared directlyin the present process byusing other basic salts or reactants in thescrubbing liquid, or by appropriate subsequent treatment of therecovered solids. The efficiency of HF removal is satisfactorily hig'h,while the reagent consumption is low and thus very economical, thelatter result being occasioned, as will nowbeseen, by the desirably lowefliciency of S02 removal. V

It is believed that the demonstrated effectiveness of the'process can beattributed to the novel correlation of certain significant factors, asfollows. Thus in the first place,ithas' beenfound that hydrogen fluorideis about three thousand times as soluble as sulfur dioxide in water. For'intan'ce, vapor-liquid equilibrium data for hydrogen fluoride and watershow that with the gas at 760 mm. pressure and zero degrees centigrade,the dissolved HF concentrationis of the 'order of 1 to 5 grams per literWhere" the equiIib'riumyapor concentration of hydrogen fluo'r'ideovertheliquid is of the order of 0.1 to 3.0 milligifa'ms percubi'c foot.Thus with the water at 40 C., thevapb'r concentration of HP is 0.5 mg.per cubic foot ove'r'wa'ter containing about 2.5 grams of per liter, andto reducethe vapor concentration to 0.1 mg. per cubic foot,'thedissolved concentrationmust be kept to about 0.5' gra rn per liter orless. Similar curves are obtainedfor other temperatures of the liquid;thus for 0.5

mg'offHF'per cubic foot in the gas, the equilibrium conceii'tration ofdissolved HE is about 1 gram per liter at 60 C. and about' 7 grams perliter at 25 C.

On the other hand, the solubility of sulfurjdi'oxide is much less? foran equilibrium concentration ofil gram of S02 per liter in watersolution, the vapor concentration (same conditions) is 500 mg., 800 mg.and about 1300 mg. per cubic foot over Water at 30 C., 40 C. and 50 C.respectively. Likewise to reach a dissolved concentrat'ion of even 0.2grams of sulfur dioxide per liter,.the equilibrium vapor concentrationis from about 70 to 140' milligrams per cubic foot. While these figuresshow that. hydrogen fluoride is many times more soluble than sulfurdioxide, the removal of hydrogen fluoride by water absorption alone isuneconomical, in that the dissolved HF concentration is nevertheless toolow for efiicient subsequent recovery. If, however, an alkaline solutionis utilized to increase the fluoride absorption, the sulfur dioxide isalso avidly taken up by reaction with the base and the dilfe'rence insolubility of the gases has no useful effect; the only significantresult is the conversion of both .HF and S02 to corresponding salts,rather than the limitation of'either HP or S02 to its presence in thefree dissolved state alone.

In the present process the scrubbing liquid contains a neutralizingreagent but is maintained at an acid rather than an alkaline pH. It hasbeen found that under these circumstances the hydrogen fluoridecontinues to react significantly while the sulfur dioxide reactsrelatively very little with the neutralizing compound. Thus the S02takeup tends to be limited, in fact, by the low solubility of such gasin the free state compared to the solubility of HF, while at the sametime the removal of the HF is vastly enhanced by chemical absorption.

That is to say, by restricting the amount of alkaline material presentso that there is never enough or never more than enough to react withthe hydrofluoric acid available, there is relatively small opportunityfor S02 absorption by chemical reaction. Indeed such absorption isspecifically limited in the presence of free, dissolved hydrogenfluoride, since fluorine or fluorine ions (as in HE) will tend todisplace 502 from any salt which the latter tends to form. Inconsequence the alkaline or neutralizing material is consumedpreferentially by the HF and any continuingSOz pick-up in the process isessentially limited to no more than is occasioned by the fresh watermake-up in recirculation. A highly eflicient separation of hydrogenfluoride is therefore achieved, and likewisecontinuous conversion of thefluoride values to reclaimable form, while the absorption efiiciency forsulfur dioxide is kept desirably low and reagent consumption isrestricted to about the actual requirements of fluoride Theaccompanyingdrawing is a diagrammatic view of asystem of apparatus,providing a flow diagram of one mple. ft e P O,cess.,,

While other arrangements and sequences of steps can be used, the systemshown affords an effective mode of operation, utilizing a spray-typescrubber 10 and other associated instrumentalities, which may all be ofconventional construction and are therefore not illustrated in detail.Thewaste or other gas to be treated enters the scrubber 14) at the inlet11, and contains both hydrogen fluoride and sulfur dioxide. The treatingliquid, containing some HP from previous circulation and containing soda(sodium carbonate) in amount'insuflicient to raise thepH above 5.5, butusually enough to reach a pH of about. 3.5 to 4.0,, is supplied to thescrubber sprays at 12,ias'likewise"n1ake-up water (at 13) to replaceevaporation losses. The acid liquor absorbs hydrogen fluoride from thegas, selectively relative to sulfurdioxide as ex-.

plained above, and the scrubbed gas is discharged at 14. The absorbed HFappears in the liquor, (a) in part as a greater dissolved concentration(lowering the pH), and (b) usually in greater part in reacted form, e.g. as sodium fluoride. Since the solubility. of the latter is very lowin these acid ranges of pH, it in'efiect precipitates as it forms, sothat the liquid discharged at 15 from the scrubber is aslurry.

The slurry is conducted, as by a pump 16, to a settling tank 17 fromwhich the precipitated fluoride, e. g. NaF, can be withdrawn at thebottom 18 while a pump 19 continuouslyre't'urns-the overflow, i. e. theclear liquor, to thel scrubber at 12/ Make-up soda solution from asupply; tank 20 is added continuously or intermittently to therecirculating line by a pump 21, at any convenient point;.fdrexample,"the soda can be eflectively added to the slurry. passing'to'the settling tank, with some advantage" of" precipitating ifu'rtherfluoride for separation before recirculatioriof the liquid through thescrubber. As'explained,the soda addition need only be controlledtomaintai'n a desired 'pH or pH range in the liquid, and thusi'n mariyc'as'e's'ca'n be regulated automatically, as by a suitable pHcontroller" 22"which'is' responsive to the condition of the 'slu1fryleaving the scrubber at 15 and which adjusts the pump '21' or'othcr flowcontrol means so that the rate of soda addition is just sutficient torestore the pH to the value desired at 12. For instance, in a typicaloperation wherein it is desired to scrub the gas with liquid at pH 3.5to 4.0, the slurry eflluent from the scrubber may show a pH of 1.0, dueto the expected absorption of HF by solution andreaction. The controller 22 then sets the rate of soda addition at a point which willraise the pH, say, to 4.0 in the slurry and therefore in the clarifiedsolution returned to constitute the scrubber spray. In some instancesthe scrubbing solution may be highly buffered so that automatic pHcontrol by known methods is diflicult. In those situations, resort maybe had to analytical control by titration for free acid.

i In many cases the pH control need not be very precise. Indeed liquidshaving a pH'approaching the lower limit of about 1.0 may still exhibitsome scrubbing efiiciency for HF, while even at a pH as 'high as 5.5 theliquid is sufliciently acid for significant suppression of S02absorption. Best results, however, are usually achieved by endeavoringto keep the liquid, which is presented to the gas, at a selected valueor range above 1.5 and below 5.0, present evidence being that a mosteffective operating point is the range of 3.5 to 4.0 or 4.5. 7

In general, actual operating conditions can be varied to suit thecircumstances and the results desired. With the present process, highscrubbing efiiciencies for hydrogen fluoride, rneasured as percentage offluorine removed, are readily obtainable in most cases, i. e. anefiiciency of 90% or, more. The actual content of HF remaining in thegas after a scrubbing treatment depends, however, on the initialconcentration as well as on the efliciency; Whether in a given case asingle scrubbing stage is adequate for safety in the ultimate release ofgas to the atmosphere or whether one or more additional scrubbing stageswill be needed, depends also, of course, on the total rate of gasdischarge. The present description is directed simply to a single-stagecontinuous operation, as being usually sufficient, or indeed asillustrating each step in a sequence where such is necessary.

The process is eifective over a wide range of hydrogen fluorideconcentrations in the gas, successful tests having been made with gasescontaining as little as 0.4 to 1.5 milligrams of fluorine per cubic foot(at normal temperature and pressure), and as high as to 60 milligrams offluorine per cubic foot. For efficiencies of 90% or better, i. e. toreduce the fluorine content to one-tenth or less of its initial value inthe exit gas, the permissible free HF concentration in the liquor maycorrespondingly vary. Thus if the exit vapor concentration is to be lessthan 0.1 mg. per cubic foot, the free HF content in the liquor shouldnot exceed about 0.5 grams per liter at 40 C. For inlet gas at thehigher end of the concentration range mentioned above, good scrubbingefficiency (not more than 30mg. of fluorine per cubic foot in the exitgas) can be obtained with free HF concentrations up to 5 grams per literin the liquor. The liquor temperature should be kept reasonably low,usually in the range of 20 to 60 C., and preferably not more than about45' C; when a gasrelatively dilute in HF is under treatment. Asindicated, the required acid condition of the liquor is achieved bymaintenance of a small but definite free HF concentration at all times;for example, such concentration can range from 0.1 to 0.5 grams perliter (or sometimes up to 2 g. p. l.) where the treated gas containsonly about 1 mg. of hydrogen fluoride per cubic foot, and can be from0.2 to 5.0 grams per liter for gases of substantially greater fluoridecontent, e. g. up to 60 mg. per cubic foot.

While acid scrubbing for effective separation of hydrogen fluoride froma gas can be accomplished with'a liquid which is primarily an aqueoussolution of any substance that is at least somewhat water soluble, thatreacts with HF in water toform a non-gaseous product and that preferablygives, an alkaline reaction whendissoly'ed in pure water, a specialfeature of the invention i's the use of a substance such as sodiumcarbonate as the neutralizing ree agent. Not only is sodium carbonateparticularly effective for the described purpose and especially forproduction of the corresponding fluoride which eventually precipitatesout as it forms, but there is nobuild-up of undesirable constituents inthe liquor, i. e. in that the carbonate ion disappears on reaction.Excellent results are similarly obtained with sodium aluminate, andother substances of like utility are soluble carbonates and hydroxides,for example of alkali metals (including ammonium), the carbonates andalurninates being preferable as more conveniently susceptible of thedesired pH control.

In some instances materials such as aluminum hydroxide (in the form ofso-called alumina gel) can be employed, and also calcium or magnesiumcarbonates, although there is apt to be some procedural inconvenience inthe use of substances having limited or little solubility. Otheralkaline salts can be employed in an acid scrubbing process, althoughsuch salts may lack the special advantages of sodium carbonate and thelike and may therefore allow accumulation of undesired radicals oranions, which complicate or impede ultimate recovery of fluoride. Suchcompounds, however, may nevertheless be useful, at least where theprimary concern of the process is to purify the gas of its hydrogenfluoride content, with less regardfor economical recovery ofuncontaminated fluoride.

especially alkali metal or other soluble compounds of this type.

As will be appreciated, the selection of the neutralizing reagent in thepreferred type of operation may be somewhat dictated by the nature ofthe fluoride compound sought to be directly recovered; with sodiumcarbonate, for example, the immediate precipitated product is usuallysodium fluoride, while cryolite or aluminum fluoride can be madedirectly by incorporating sodium aluminate or aluminum hydroxide,respectively, in place of soda.

Subsequent treatment of the separated sodium fluoride or other fluoridematerial can be relatively simple, there being usually no need for thecarbon dioxide gassing operation heretofore required with the liquorfrom alkaline scrubbing. As indicated above, cryolite or otheraluminum-containing fluoride can be produced directly it desired, or ifgreater convenience is achieved by the use of soda in the scrubbingliquor, and yet such compounds are ultimately sought, the sodiumfluoride can be suitably treated as precipitate or in solution, e. g. byreaction with aluminate, to yield the desired product. As will beappreciated, the nature of the further recovery treatment, which is notan essential part of the present invention, may vary, too, with thepurity of the separated precipitate or solution. For example, if solidsin the waste gas have been removed before scrubbing, the present processmay be employed to yield a relatively pure precipitate of sodium orother desired fluoride; alternatively, if the dust material is alsoaccumulated in the scrubbing liquor and withdrawn with the precipitate,subsequent treatments may require dissolving the fluoride, as in thecourse of conversion to a different and preferred chemical form.

In one example of operation of the process, carried out on a benchscale, the gas under treatment was first filtered to remove suspendedsolids and was then scrubbed by bubbling through the acid liquor in asuitable tank. The gas flow was 0.3 cubic feet per minute (considered atnormal temperature and pressure) and averaged 38.5

mg. of fluorine (in the form of hydrogen fluoride) and 106 mg. of sulfurdioxide, per cubic foot. Soda (sodium carbonate) was used as the reagentin the scrubbing liq-.

nor, the latter being an aqueous solution kept at a pH of 3.5 byadditions of soda from time to time. Combined fluorine continuouslyprecipitated out inthe liquor, the

product in this instance being sodium fluosilicate rather than sodiumfluoride, because of reaction with the glass tank employed forscrubbing. Where a suitable rubber- Examples of such alkaline salts arephosphates and borates.

- lined orother acid-proof scrubbing tank is used, the'prepitat is s d ufluQr The test of this example was continued for approximately 90 hours,and resulted in an over-all fluorine recovery efliciency of 95.5%, witha soda consumption tota ling 130.5 grams and equivalent to 2.03 tons ofsoda per ton of fluorine recovered. The sulfur dioxide scrubbingefficiency was desirably low, i. e. 30.2%. The saturation concentrationsreached in the clear scrubber liquor were 3.5 grams of fluorine perliter and 4 grams of S02 per liter, while the average content offluorine and S02 in the exit gas from the scrubber amounted respectivelyto 1.7 and 74 mg. per cubic foot. During this actual run, soda was addedat an average rate of 24.6 mg. per minute, it being understood that inpreferred practice the control need be related only to the pH oftheliquor, and need not require any specific predetermination of theamounts of soda added or present. While in the relatively short time ofthis test operation, the actual distribution of total absorbed fluorinewas 44.5 grams in the clear liquor (partly as free HF but mostly asdissolved fluorine salt) and 19.6 grams in the recovered solids, it willbe understood that over a long-continuing run of this type of operation,the principal, and indeed effective recovery of fluorine is in'thesolids,'usually as precipitated in the scrubber or in the course ofrecirculation of liquor leaving the latter.

As will be seen, the test showed a desirably high efficiency of hydrogenfluoride scrubbing, with relatively small reagent waste in removingsulfur dioxide, the large preponderance of the latter being carriedthrough without absorption. The soda consumption of about 2 tons per tonof recovered fluorine is remarkably low in comparison with prioralkaline scrubbing operations, in which 5.5 tons of soda have beenrequired per ton of fluorine recovered in the clear liquor.

Another example of the process is represented by a plant scale test,which was carried out for 8 hours with a conventional acid-proofscrubber. The liquor (primarily an aqueous solution of sodium carbonate)was fully recirculated, while a pH of about 4.8 was maintained, withappropriate extra soda. The hydrogen fluoride content of the gas treatedwas about 7 to 13 milligrams per cubic foot, and the free HFconcentration in the scrubbing liquor was effectively controlled at notmore than about 2.5 grams per liter, i. e. thus to prevent theequilibrium concentration of HP in the scrubbed gas from exceeding aboutof its value in the original gas. During'the 8 hour run, the totalfluorine entering the scrubber in both gas and dust was about 400pounds, with a total of only about 23 pounds of fluorine leaving thescrubber; a total of 368 pounds of fluorine was recovered, being 194pounds in the liquor and 174 pounds in the scrubber solids. The fluorineconcentration in the liquor built up to 20 grams per liter in 3 hoursand remained at that value, which corresponds to the solubility limit ofsodium fluoride, for the remaining time; thus as indicated above, afterequilibrium conditions are reached in the scrubbing circuit, actualremoval and recovery of fluoride is essentially confined to itsprecipitation as sodium fluoride or equivalent compounds.

In this plant scale test the total sodium sulfate pickup was only 211pounds, corresponding to an S02 removal efliciency of26%,which'represents great improvemerit over the undesirably high eflieiencyof 97% that was obtained in alkaline scrubbing procedure on similarwaste gas. The total soda consumption was only a minor fraction of thatrequired in the conventional alkaline process.

It-will; be understood that'the above examples are merely illustrativeand that the process is applicable to a wide variety of circumstances,for separation of hydrogenfluoride, and'especially for economicalseparation and recovery'of fluoride from gases which'also containsubstantial quantities of sulfur dioxide.

While in the presently preferred operations described above the fluorinerecovery has been emphasized as involving direct precipitation of sodiumfluoride or the like, it will be understood that the liquor alwaysbuilds up to a saturation concentration of dissolved sodium fluoride; incontinuing the above specific process utilizing soda as reagent thisdissolved fluoride salt concentration simply remains and at most onlycontributes in a transient sense to the continuing fluoride recovery byprecipitation. In other cases, however, e. g. especially where otherreagents may be used in the scrubber liquor, considerably higherconcentrations of dissolved fluorine salts may possibly be reached (asdistinguished from dissolved, free HF), and the recirculating processmay then conceivably include special precipitating or equivalent stepsfor reducing the fluorine salt content, i. e. for separating andrecovering the desired product.

It is to be understood that the invention is not limited to the specificembodiments herein shown and described but may be carried out in otherways without departure from its spirit.

I claim:

1. A process of removing hydrogen fluoride from aluminum reduction cellgas which contains combined fluorine that consists essentially ofhydrogen fluoride, and which also contains sulfur dioxide in significantamount, comprising scrubbing the aforesaid gas with aqueous liquid whichis subjected to recirculation for succeeding scrubbing operations, saidliquid being a dilute acidic solution having a pH in the range of 1.0 toabout 5.5 in which the acidic component consists essentially ofdissolved hydrogen fluoride and in which an alkaline neutralizing agenthas been introduced, introducing successive quantities of alkalineneutralizing agent into said liquid in the course of recirculation tomaintain said liquid at a pH within the aforesaid range during thescrubbing operations, said introductions of neutralizing agent into theliquid and said operations of scrubbing the aforesaid cell gas with theliquid being performed to convert hydrogen fluoride from said gas intothe form of fluoride salt in said liquid, said fluoride salt being areaction product of said neutralizing agent and hydrogen fluoride, andtreating successive quantities of the liquid to recover fluoridetherefrom, for removal of fluoride salt from the course ofrecirculation, said process including sutficiently limiting theintroductions of neutralizing agent and maintaining sufficientconcentration of dissolved hydrogen fluoride by absorption from the gas,to keep the pH of the liquid substantially no higher than the aforesaidrange, for inhibiting absorption of sulfur dioxide in the liquid.

2. A process as described in claim 1, wherein the alkaline neutralizingagent comprises a soluble alkaline salt of an alkali metal.

3. A process as described in claim 1, wherein the alkaline neutralizingagent comprises sodium carbonate.

4. A process as described in claim 1, wherein the alkaline neutralizingagent comprises sodium aluminate.

5. A process as described in claim '1, wherein the alkaline neutralizingagent comprises sodium carbonate, reacting in the scrubbing liquid toprecipitate sodium fluoride, the introductions of neutralizing agentinto the liquid being controlled to provide a pH at least as high asabout 3 in the liquid as it enters the scrubbing operation, and thetreatment of successive quantities of the liquid to recover fluoridecomprising separating precipitated sodium fluoride from therecirculating liquid intermediate successive scrubbing operations.

6. A process asdescribed in claim 1, wherein the introductions ofneutralizing agent are controlled to maintain a pH in the liquid withinthe range of about 1.5 to about 5 during the scrubbing operations.

7. Aprocess of removing hydrogen fluoride from alumin m i T1 I S w ichss ta n mb n u r n that consists essentially of hydrogen fluoride, andwhich alkaline agent selected from the class consisting of alkali metalaluminates and the carbonates and hydroxides of alkali metals, aluminum,calcium and magnesium, introducing successive quantities of the alkalineagent into saidliquid in the course of recirculation to maintain saidliquid at a pH within the aforesaid range during the scrubbingoperations, said introductions of alkaline agent into the liquid andsaid operations of scrubbing the aforesaid cell gas with the liquidbeing performed to convert hydrogen fluoride from said gas into the formof fluoride salt precipitating invsaid liquid, said fluoride salt beinga reaction product of said alkaline agent and hydrogen fluoride, andseparating successive quantities of said precipitated fluoride salt fromthe recirculating liquid intermediate successive scrubbing operations,the aforesaid introductions of alkaline agent being limited to keep thepH of the liquid substantially no higher than the aforesaid range, forinhibiting absorption of sulfur dioxide in the liquid.

8. A process as described in claim 7, wherein the introductions ofalkaline agent into the liquid are controlled to provide a pH as high asabout 3 in the liquid as it enters the scrubbing operation.

References Cited in the file of this patent UNITED STATES PATENTS1,324,030 Bowman Dec. 9, 1919 1,464,990 Howard Aug. 14, 1923 2,031,554Torchet Feb. 18, 1936 2,231,309 Weber Feb. 11, 1941 2,385,208 JonesSept. 16, 1945 2,597,302 Dale May 20, 1952

1. THE METHOD OF PRODUCING SULFUR DIOXIDE IN CONALUMINUM REDUCTION CELLGAS WHICH CONTAINS COMBINED FLUORINE THAT CONSISTS ESSENTIALLY OFHYDROGEN FLUORIDE, AND WHICH ALSO CONTAINS SULFUR DIOXIDE IN SIGNIFICANTAMOUNT, COMPRISING SCRUBBING THE AFORRESAID GAS WITH AQUEOUS LIQUIDWHICH IS SUBJECTED TO RECIRCULATION FOR SUCCEEDING SCRUBBING OPERATIONSSAID LIQUID BEING A DILUTE ACIDIC SOLUTION HAVING A PH IN THE RANGE OF1.0 TO ABOUT 5.5 IN WHICH THE ACIDC COMPONENT CONSISTS ESSENTIALLY OFDISSOLVED HYDROGEN FLUORIDE AND IN WHICH AN ALKALINE NEUTRALIZING AGENTHAS BEEN INTRODUCED, INTRODUCING SUCCESSIVE QUANTITIES OF ALKALINENEUTRALIZING AGENT INTO SAID LIQUID IN THE COURSE OF RECIRCULATION TOMAINTAIN SAID LIQUID AT A PH WITHIN THE AFORESAID RANGE DURING THESCRUBBING OPERRATIONS, SAID INTRODUCTIONS OF NEUTRALIZING AGENT INTO THELIQUID AND SAID OPERATIONS OF SCRUBBINNG THE AFORESAID CELL GAS WITH THELIQUID BEING PERRFORMED TO CONVERT HYDROGEN FFLUORIDE FROM SAID GAS INTOTHE FORM OF FLUORIDE SALT SAID LIQUID, SAID FLUORIDE SALT BEING AREACTION PRODUCT OF SAID NEUTRALIZING AGENT AND HYDROGEN FLUORIDE, ANDTREATING SUCCESIVE QUANTITIES OF THE LIQUID TO RECOVER FLUORIDETHEREFROM, FOR REMOVAL OF FLUORIDE SALT FROM THE COURSE OFRECIRCULATION, SAID PROCESS INCLUDING SUFFICIENTLY LIMITING THEINTRODUCTIONS OF NEUTRALIZING AGENT AND MAINTAINING SUFFICIENTCONCENTRATION OF DISSOLVED HYDROGEN FLUORIDE BY ABSORPTION FROM THE GAS,TO KEEP THE PH OF THE LIQUID SUBSTANTIALLY NO HIGHER THAN THE AFORESAIDRANGE, FOR INHIBITING ABSORPTION OF SULFURDIOXIDE IN THE LIQUID.